Input/Output

C - Input and Output

Input

 In any programming language input means to feed some data into program. This can be given in the form of file or from command line. C programming language provides a set of built-in functions to read given input and feed it to the program as per requirement.

scanf() function

This is the function which can be used to to read an input from the command line.
Try following program to understand scanf() function.

#include <stdio.h>

main()
{
  int x;
  int args;

  printf("Enter an integer: ");
  if (( args = scanf("%d", &x)) == 0) {
      printf("Error: not an integer\n");
  } else {
      printf("Read in %d\n", x);
  }
}

Output : 

In any programming language output means to display some data on screen, printer or in any file. C programming language provides a set of built-in functions to output required data.
Here we will discuss only one input function and one putput function just to understand the meaning of input and output.

printf() function

This is one of the most frequently used functions in C for output. ( we will discuss what is function in subsequent chapter. ).
Try following program to understand printf() function.

#include <stdio.h> main() { int dec = 5; char str[] = "abc"; char ch = 's'; float pi = 3.14; printf("%d %s %f %c\n", dec, str, pi, ch); }

The output of the above would be:

5 abc 3.140000 c

Here %d is being used to print an integer, %s is being used to print a string, %f is being used to print a float and %c is being used to print a character.
A complete syntax of printf() function is given in 

Here %d is being used to read an integer value and we are passing &x to store the vale read input. Here &indicates the address of variable x.
This program will prompt you to enter a value. Whatever value you will enter at command prompt that will be output at the screen using printf() function. If you enter a non-integer value then it will display an error message.

Enter an integer: 20
Read in 20

 

puts() vs printf() for printing a string

In C, given a string variable str, which of the following two should be preferred to print it to stdout?

   1)  puts(str);

   2)  printf(str);

puts() can be preferred for printing a string because it is generally less expensive (implementation of puts() is generally simpler than printf()), and if the string has formatting characters like ‘%’, then printf() would give unexpected results. Also, if str is a user input string, then use of printf() might cause security issues (see this for details).
Also note that puts() moves the cursor to next line. If you do not want the cursor to be moved to next line, then you can use following variation of puts().

   fputs(str, stdout)

You can try following programs for testing the above discussed differences between puts() and printf().

Program 1

#include<stdio.h> int main() {     puts("allintutor");     puts("allintutor");           getchar();     return 0; }

Program 2

#include<stdio.h> int main() {     fputs("allintutor", stdout);     fputs("allintutor", stdout);           getchar();     return 0; }

Program 3

#include<stdio.h> int main() {     // % is intentionally put here to show side effects of using printf(str)     printf("allintutor%sforallintutor%s");      getchar();     return 0; }

Program 4

#include<stdio.h> int main() {        puts("allintutor%sallintutor%s");        getchar();     return 0; }

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

 Scansets in C

scanf family functions support scanset specifiers which are represented by %[]. Inside scanset, we can specify single character or range of characters. While processing scanset, scanf will process only those characters which are part of scanset. We can define scanset by putting characters inside squre brackets. Please note that the scansets are case-sensitive.

Let us see with example. Below example will store only capital letters to character array ‘str’, any other character will not be stored inside character array.

/* A simple scanset example */
#include <stdio.h>

int main(void)
{
    char str[128];

    printf("Enter a string: ");
    scanf("%[A-Z]s", str);

    printf("You entered: %s\n", str);

    return 0;
}

  [root@centos-6 C]# ./scan-set 
  Enter a string: GEEKs_for_geeks
  You entered: GEEK

If first character of scanset is ‘^’, then the specifier will stop reading after first occurence of that character. For example, given below scanset will read all characters but stops after first occurence of ‘o’

 scanf("%[^o]s", str);

Let us see with example.

/* Another scanset example with ^ */
#include <stdio.h>

int main(void)
{
    char str[128];

    printf("Enter a string: ");
    scanf("%[^o]s", str);

    printf("You entered: %s\n", str);

    return 0;
}

  [root@centos-6 C]# ./scan-set 
  Enter a string: http://geeks for geeks
  You entered: http://geeks f
  [root@centos-6 C]# 

Let us implement gets() function by using scan set. gets() fucntion reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF found.

/* implementation of gets() function using scanset */
#include <stdio.h>

int main(void)
{
    char str[128];

    printf("Enter a string with spaces: ");
    scanf("%[^\n]s", str);

    printf("You entered: %s\n", str);

    return 0;
}

  [root@centos-6 C]# ./gets 
  Enter a string with spaces: all in tutor
  You entered: all in tutor
  [root@centos-6 C]# 

What is return type of getchar(), fgetc() and getc() ?

In C, return type of getchar(), fgetc() and getc() is int (not char). So it is recommended to assign the returned values of these functions to an integer type variable.

  char ch;  /* May cause problems */  
  while ((ch = getchar()) != EOF) 
  {
     putchar(ch);
  }

Here is a version that uses integer to compare the value of getchar().

  int in;  
  while ((in = getchar()) != EOF) 
  {
     putchar(in);
  }

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